We have been enjoying an amazingly warm and sunny late winter and spring so far in 2013. As I go around the vineyards, the classic refrain from the Broadway musical Hair is playing in my mind. The vines leafed out in early April (which is early for us), there is not too much moisture in the ground (which is good for us given that excess vigor is our main viticultural challenge each vintage) and 2013 is likely going to be a ripe vintage.

I recently did some calculations about how much UV light our vines receive each growing season compared to vines in the surrounding valleys. First, most of our vineyards (74% to be precise) are on the Sonoma side of the county line, which happens to be the crest of the Mayacamas mountains that wanders through our property. Those Sonoma blocks face nearly due south and slope from 5 to 9 degrees. At our latitude, the sun is not straight overhead at noon, so our southern facing exposures mean the solar intensity is increased compared to a perfectly level vineyard. When averaged for the sun’s position in the sky between June 1 and September 1, a bit of trigonometry reveals that our vineyards receive an additional 12% of sunlight compared to a level vineyard at our latitude. Further, because we are 2000 feet higher in elevation compared to the surrounding valleys, the sunlight is 8% stronger because there is less atmosphere for the light to pass through. Additionally, because we are almost always above the fog layer that invades the valleys most summer evenings, we begin the day in sunshine. This provides about 8% more UV light than valley vineyards that often start their day in fog. Last, being on a mountaintop means we have no morning or afternoon shadows, which results in roughly 2% more UV compared to a typical valley vineyard. Adding up, we receive about 30% more UV light each growing season due to our mountaintop location.

There is a temperature inversion most days in which the higher you go, the warmer it gets. During the growing season, the typical overnight low in our highest blocks is about 70 degrees F, which is much warmer than the 54 degrees inside the fog layer in the valleys. So as soon as the sunlight hits our vines each morning, the vines are already at a temperature where growth and photosynthesis are active. On that typical summer day, the temperature will only rise 15 degrees to a high of 85. Overall, our average temperature is usually cooler than the valleys during the growing season.

It is pretty clear that our enhanced sunlight and warm morning temperatures are the main reasons our red wines develop such intense phenolic concentration. Phenolic molecules give red wine its color and structure. These molecules come dominantly from the grape skins and grape seeds. The long-chain tannin molecules that are responsible for much of the weight and mouthfeel of red wine come exclusively from the grape skins and are significantly enhanced in grape clusters exposed to higher levels of UV light. We typically have 1000 to 1300 mg/liter of long-chain tannin molecules in our cabernet sauvignon, which can be compared to 400 to 800 mg/liter for cabernet from a typical valley vineyard. So a good part of our “mountain” character is directly correlated to the extra sunlight we receive. The phenolic molecule quercetin is produced in the grape’s skin to protect the grape from UV light. We have high levels of quercetin in our wines. For my skin, I need to apply a strong sunblock when I am out in the vineyards. So coated with our respective sunscreens (quercetin and Coppertone), the vines and I are ready to “Let the Sunshine In” for the rest of 2013.

Steve Pride